This invention relates to semiconductor devices as typified by insulated gate bipolar transistors (IGBTs), and particularly to those having guard trenches, in addition to cell trenches, etched into the semiconductor substrate for higher voltage strength. The invention also specifically pertains to a method of fabricating such trench semiconductor devices.
IGBTs have been known which are cell-trenched to withstand higher voltages, as disclosed for example in Japanese Unexamined Patent Publication No. 9-283754. The cell trenches accommodate gate electrodes via insulators.
The same unexamined patent application also teaches how to improve the voltage strength of the IGBT at the periphery of the semiconductor substrate around the group of IGBT trench cells placed centrally thereon. Employed to this end are several annular guard trenches arranged concentrically along the substrate periphery so as to surround the cell cells. Each guard trench receives a guard trench conductor via an insulator (dielectric). All the guard trench conductors are electrically interconnected via semi-insulators. Like the more conventional guard ring technology, the guard trenches with the guard trench conductors therein function to mitigate field concentrations at the substrate periphery, where the pn junction of the semiconductor device terminates, and hence to enable the device to withstand higher voltages.
The guard trench structure possesses some distinct advantages over the guard ring technology. First of all, in cases where relatively deep cell trenches are etched centrally in the semiconductor substrate for switching applications, the guard rings must be of matching depth. However, being created by diffusion of a conductivity type determinant into the semiconductor substrate, such deep guard rings become unnecessarily wide as a result of inevitable lateral, in addition to desired depth-wise, diffusion of the conductivity type determinant. The semiconductor chips must be made correspondingly larger in size in order to accommodate such wide guard rings. This inconvenience does not occur with the guard trenches, which are capable of creation by known anisotropic etching to a much less width than the guard rings.
Second, formed as above by impurity diffusion, each guard ring becomes either semicircular or elliptical in cross sectional shape. The deepest parts of such guard rings are spaced an unnecessarily long distance away from the cell trenches placed centrally of the semiconductor substrate. The depletion layer is not formed in some such spacings, with a consequent failure in mitigation of field concentrations. The anisotropically etched guard trenches, on the other hand, extend in their depth direction almost at right angles with the substrate surface. The depletion layer is created adjacent the bottoms of the guard trenches for most effective alleviation of field concentrations.
Third, the fabrication of guard rings by impurity diffusion necessitates the semiconductor substrate to be held at high temperatures for prolonged lengths of time. The deeper the guard rings, moreover, the longer must the semiconductor substrate be held heated. Such prolonged heating of the semiconductor substrate brings about an undesired impurity diffusion from one part to another of the substrate. Let us consider an IGBT for instance. An undesired impurity diffusion will occur from n-type buffer region to n−-type base region of the IGBT, to such an extent that the latter region will become unnecessarily high in impurity concentration. The base region with such high impurity concentration will make it difficult for the depletion layer to spread therethrough. The device will then fail to withstand as high voltages as desired. No such prolonged heating is required, and no such undesired impurity diffusion occurs, for creation of the guard trenches complete with the conductors and insulators received therein. Higher voltages can therefore be normally tolerated with the guard trenches than with the guard rings.
Despite all these advantages over the guard rings, the guard trenches as hitherto created possessed some difficulties left unremedied. The guard trenches enable the semiconductor device to withstand a variable voltage depending upon their exact depths and spacings. The creation of the guard trenches to stringent dimensional and positional specifications has therefore been so far essential for provision of semiconductor devices capable of withstanding desired high voltages. Currently, however, there exist limitations that are still insurmountable for fabricating guard trenches to sufficiently close tolerances to provide such rugged semiconductor devices.
The problem of insufficient or unreliable voltage strength at the periphery of the semiconductor substrate is not limited to IGBTs. It has existed with insulated gate transistors other than IGBTs, as well as comparable controllable solid-state switches, thyristors, diodes, and other trench semiconductor devices.